Posted by: Li Ling Young | March 20, 2016

Your House is a Battery

(originally posted in the Vermont Zero Energy Home Pilot blog, where I write as a building science professional, rather than as a homeowner)

We used to get this question a lot, “Do I really save energy by letting my house cool off when I’m at work, since the heating system has to work so hard to warm the house back up later?”  Vermonters are frugal and it’s common to use a temperature “setback”: allowing the house to be cooler than is comfortable when no one is home, or when everyone is asleep. But, there is skepticism that it doesn’t actually save energy since warming the house back up (surely) uses so much energy.

My stock answer to this question is, Yes.  Courtesy of my colleague, Paul Scheckel, an analogy: When you want to make tea, do you keep the kettle hot all day, only to pour your orange pekoe at 4pm?  No, you turn the kettle on when you want the water hot.  The moral: there is no need to keep your house warm all the time if you’re only going to enjoy that warmth for a few hours in the morning and a few at night.  Caution, I’m going to contradict this later on: DON’T TAKE MY WORDS OUT OF CONTEXT.

Sometimes my questioner shoots back, “But my house doesn’t ever get down to 62 (or whatever the setback temperature is) degrees.”  The purpose of the setback isn’t to make the house colder, it’s to prevent the thermostat turning on the heating system (as frequently) during the setback period.

But why didn’t the house cool off?  Because it’s insulated.  Teakettles aren’t insulated.  The better insulated and air sealed a house is, the more slowly it cools off.  Very air tight and well-insulated homes cool off so slowly that they only call for heat infrequently, whether or not the house is in setback.  Efficient homes save less from thermostat setback than poorly-insulated and leaky homes.   Older, poorly-insulated homes can save quite a  bit of energy by using thermostat setbacks, and that’s probably why this strategy has taken hold in the Vermont imagination as a way to be frugal.

The reason heating the house back up after setback doesn’t incur an energy penalty is that most heating systems operate at the same efficiency no matter how much heating they have to do.  In fact some heating systems, particularly oil -burners, are a little more efficient when they run for a long time, as they would when warming the house back up.

Now comes a new type of heating system – the cold climate air source heat pump.  (Also occasionally known as: ductless heat pump, mini-split, heat pump, air source heat pump…)  The defining feature of a cold climate heat pump is that it’s variable speed; it doesn’t operate only in on-mode or off-mode, it can put out a little bit of heat or a lot of heat.  Cold climate air source heat pumps are most efficient when they are not working very hard: very much like a car rolling along at a constant speed.

Also unique to cold climate heat pumps is that they make less heat as the temperature outside drops.  So, if you combine the lower efficiency and lower output at low temperature you can see that you don’t want to ask your heat pump to do a lot of hard work when it’s very cold outside.

heat pump capacity temp

As the outdoor temperature (horizontal axis) drops, an air-source heat pump loses capacity to make heat.  This model only makes about 70% of it’s rated capacity (vertical axis) at -13 deg F.  As a side note, these heat pumps have been observed making plenty of heat at -24 deg F.  Cold climate heat pumps work great, but you have to size them properly for your climate and your house.

So how should one operate a house heated with a cold climate heat pump?  Do not set the temperature back, ever.  By keeping the house at the same temperature it’s like driving a car at a constant speed on a flat road: you only have to put a little energy in to keep going.  On the other hand, recovering from a setback period (changing the temperature of the house) is like accelerating a car uphill: you’re asking the car to work hard, it does so less efficiently AND your car has less power the steeper the hill is.  The car won’t be able to do what you’re asking , and it will use a lot of gas trying.  Or, to shed the analogy, your house won’t recover from the setback very quickly, and it will do so at the lowest efficiency.  So don’t accelerate and don’t drive uphill.

What about the teakettle?  Isn’t it wasteful to keep the house toasty warm when no one is home?   Remember, well-insulated and air-sealed homes don’t gain much from letting the house cool off.  In fact, given the unique nature of a cold climate heat pump it might actually make the most sense to heat the house up when no one is home.  If the warmest part of the day is at 1pm, when everyone is at school or the office, it might be best to ask your heat pump to get the house extra warm at that time, when it can do so at a higher efficiency, rather than expecting the heat pump to do a lot of heating work at 5:00 when everyone is home but the temperature has dropped 10 or 20 degrees.  But ONLY if the house is well-insulated and air-sealed so it can hold onto that heat energy long enough that you get to enjoy it later.

This is how a well-insulated house is like a battery.  You can fill it up with heat energy at the best time, and use the heat energy later when you want to be comfortable at home.  It only works if your house can hold the heat, and it’s only beneficial if you’re using a cold climate heat pump.

I have two purposes in explaining all this:

  1. Point out how important both building improvements and advanced heating systems are; and how they work together to make a whole greater than the sum of the parts
  2. Make a compelling case for why one operates a heat pump-heated home differently from a fuel-heated home.

Here’s my best advice on how to operate your heat pump for best efficiency and comfort

  • Set the temperature you want the house to be and leave the thermostat there all season (winter operation; in the summer just turn the AC on when you want it)
  • Leave all the interior doors open when no one is home (good idea all the time, but desire for privacy may trump this)
  • In very cold weather you can get more heat out of the heat pump by putting it in high fan speed.  It’s noisy, so I advise doing this when you’re asleep or when no one is home (again, using the battery-function of a well-insulated home)
  • Clean the filters regularly
  • Make improvements to the insulation and air tightness of your home!
Posted by: Li Ling Young | May 13, 2015

Active House or Active People?

I find myself on the other side of the line dividing the heating-dominated climate and the cooling-dominated climate. I’m not oblivious to the cultural implications of this line, but what’s really standing out to me at the moment is the way I’m interacting with this house.

Last year a neighbor asked me about going all electric with his home, as his furnace had just been given last rites. He and his wife are professionals with two active school-aged children. My first response as I stood in the grocery isle talking to him, was a caution about how involved one wants to be with one’s home. The technologies we used to cobble together our zero energy home aren’t what I’d consider ready for primetime. Everything works just great, but nothing is integrated. And if we want to be comfortable we have to anticipate and prepare. Not everyone wants this kind of relationship with their home, especially if you have a lot of other stuff going on. At the first mention of having to keep track of the weather and make decisions about how to setup the house, my neighbor said, “Not for us.” He made a decision about his house standing there in the grocery isle that would have taken me six months of agonizing to come to.

Indeed we are the brains of our house, which probably doesn’t seem weird considering that we actually have brains and the house actually doesn’t. But most houses really do operate of their own accord, making decisions without assistance from the brain-endowed occupants. The automated systems in the house are there so we humans don’t have to make it all happen. Nonetheless, in our low-energy house we are much more involved.

We’re still on some part of the learning curve, so I wouldn’t say everything is perfect even when we are applying our brains to the house. But here’s a sample of the kinds of things we consider on a daily basis:

  • Season – we arrange the house differently depending on what season it is
  • Window position – seems obvious in the winter, but where should they be if it’s going to be 90 deg F out?
  • Sun – sunny winter day may mean we should not use the woodstove in the living room
  • Curtains – block summer sun, but not winter sun; but don’t bother if it’s cloudy
  • Temperature forecast – don’t let the house temperature drift if it’s going to be wickedly cold or hot
  • Ventilation – lots when it’s mild out, but almost none when it’s wickedly hot or cold
  • Basement temperature – do we need to change what the heat pump water heater is doing, or start using the basement woodstove to prevent the basement being too cold in the spring?
  • Snow cover – if the solar panels are covered and it’s going to be sunny, get out the roof broom!

Sound like a ridiculous amount of fussing? I’ve heard this from a building scientist, “Passive occupants need active houses, but passive houses need active occupants.” Meaning, the less you want your house to do, the more you are going to need to get involved. Ridiculous indeed if it’s a higher priority to have a house that tends to itself than a house that sips daintily at the energy well. Like my neighbors with the lightning fast decision, most really need their home to carry its own weight. And here’s yet another way our daintily-sipping house is a freakshow: Nik and I are running around like the wizard behind the curtain, pulling levers, setting up the blinking lights, adjusting angles, making the steam come out…

Here, in this other place, I find myself astonished that folks don’t know what the weather forecast is. This time of year I’m desperate for fresh air, but this house is shut up tight with the air conditioner running and the indoor humidity like a Turkish bath. Active house yes, even to the point of enforcing passivity on the occupants. When the weather took a decisive turn for the mild I begged to open the windows and got a quizzical look in return, “Should we turn the air conditioner off?”

Posted by: Li Ling Young | May 2, 2015

The Wall

We’re finally ready to start on what I consider to be the defining feature of a Zero Energy Home.  Our walls are only 4″ across, leaving precious little room for insulation.  While most folks doing energy improvements on their homes would do what they can within the wall thickness they have, to do zero energy right you generally need more than 4- or even 6″ of insulation.  Keep in mind I’m talking about the Vermont climate: about 7000 heating degree days.  In milder climates 6″ of well-installed insulation might be enough.

How do you put more insulation in a wall than will fit in a wall?  You have to make the wall thicker.  And that is why most energy improvement projects just color within the lines when it comes to the walls.  A thicker wall means you either rebuild the inside of the wall: very messy, disruptive and expensive; or you rebuild the outside of the wall: requires new siding, moving the windows, and also very expensive.  Between the two, thickening the wall to the outside is generally more effective and slightly less expensive.  Plus, it’s a chance to renew the look of your house.

The reason I say a thickened wall is a defining feature of a Zero Energy Home is that the whole zero energy thing is a little over the top at almost every level: oversized solar, super energy efficient appliances, whole home energy monitoring…  But the whole is greater than the sum of the parts, so you have to go the extra mile on all this stuff or you don’t get the zero energy special sauce.   Having superinsulated walls makes something magic happen: the house keeps itself warm.  Some day, once we get these walls done, the house will be warm everywhere, with just a little bit of heat introduced in the dining room.

Designing a thickened wall is no easy feat.  There are too many competing considerations: cost, aesthetics, effectiveness, greenness, longevity…  And there’s an overriding threat that if you do it wrong the wall will load up with water and become a giant moldy diaper draped around your house.  As an aside, we have exactly this diaper situation with the wall behind our bed.  Whenever it rains and then gets warm we can smell the house composting as we snuggle down to sleep.

Here's the problem with the wall behind our bed.  It appears that the addition has no house wrap (tar paper, or the more modern Tyvek) behind the siding.  Bad or non-existent siding at the window has allowed the wall to get wet.  We can smell the wall rotting away on rainy days.  When we thicken this wall out with more insulation and new siding, this rotten spot will have a chance to dry out and be protected from the weather.  I think it will be ok to leave it as long as we cover it up before it becomes a structural problem.

Here’s the problem with the wall behind our bed, exposed when we had cellulose pumped into the walls. The white stuff is spray foam covering the cellulose hole. It appears that the addition has no house wrap (tar paper, or the more modern Tyvek) behind the siding. Bad or non-existent flashing at the window has allowed the wall to get wet. We can smell the wall rotting away on rainy days. When we thicken this wall out with more insulation and new siding, this rotten spot will have a chance to dry out and be protected from the weather. I think it will be ok to leave it as long as we cover it up before it becomes a structural problem.

Here’s my first stab at our thickened wall design, layer-by-layer, from the inside

  • Original plaster wall – very stout and heavy: not gypsum board!
  • No plastic behind the wall board.  I don’t think plastic had been invented when this house was built.
  • Original 2X4 stud wall with a messy combination of fiberglass (original) and densepack cellulose (added two years ago) between the studs.
  • Original 3/4″ wood board sheathing.  This is the good stuff.  No plywood in this house.  Except the diaper wall on the addition.
  • Ice and Water Shield, peel-n-stick membrane.  This will be my air tight layer and my water/vapor-proof layer.  Where you put this layer is very important for diaper avoidance.  I think this is a good place to put it.
  • At this point in the wall there is currently tar paper and siding, but that will be removed.
  • A new 6″ or 8″ framed wall stuck on the outside of the building, filled with cellulose.
  • OSB – not my first choice, but my builder has talked me into it.
  • Some kind of drainage plane  house wrap: allows water behind the siding to drain away while keeping the OSB dry.
  • New siding: hopefully cedar, but maybe vinyl.

We’re getting the whole thing priced out right now.  I’m sure the design will evolve at least a little.  We’ll let you know what the price is per square foot.

An early idea about thickening the wall for more insulation had the original siding left in place.  Spray foam over the siding would seal it, and all the lead paint would be encapsulated, never to pollute again.  I like this approach because it saves the time and landfill fee of removing the siding, but my builder convinced me it makes all the subsequent construction easier if the siding is removed.

An early idea about thickening the wall for more insulation had the original siding left in place. Spray foam over the siding would seal it, and all the lead paint would be encapsulated, never to pollute again. I like this approach because it saves the time and landfill fee of removing the siding, but my builder convinced me it makes all the subsequent construction easier if the siding is removed.

 

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